The disclosure of Japanese Patent Application No. 2000-102047 filed on Apr. 4, 2000, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates generally to an improvement in the process for synthesis of lower isoparaffins from synthesis gas (hereinafter referred to as xe2x80x9csyngasxe2x80x9d when appropriate) which is a mixture of hydrogen and carbon monoxide.
2. Description of Related Art
Processes for producing lower aliphatic saturated hydrocarbons (lower paraffins) from syngas (i.e., a mixture of hydrogen and carbon monoxide) are well known in the art. An example of the known processes uses a catalyst that is a physical mixture of a methanol synthesis catalyst based on, for example, Cuxe2x80x94Zn, Crxe2x80x94Zn, Pd, or the like, with a methanol conversion catalyst comprising, for example, zeolite. In this method, the syngas is converted to lower aliphatic saturated hydrocarbons via methanol in one pass through the above-mentioned catalyst. This process for producing lower aliphatic saturated hydrocarbons via methanol, however, suffers from problems, such as severe reaction conditions, deactivation of the catalyst, and low selectivity for components whose carbon number is equal to or greater than 4 (i.e., at least C4 components).
Meanwhile, a process has been proposed for producing lower isoparaffins under relatively mild reaction conditions without causing syngas to be converted to methanol during the process. This process uses a catalyst for Fischer-Tropsch (FT) synthesis for synthesizing higher paraffins and lower olefins from syngas, and uses a solid acid catalyst, such as zeolite, for producing lower isoparaffins by hydrocracking or isomerizing the higher paraffins and lower olefins. This process for synthesis of lower isoparaffins is disclosed in xe2x80x9cDIRECT SYNTHESIS OF ISOPARAFFINS FROM SYNTHESIS GASxe2x80x9d, Kaoru FUJIMOTO et al., CHEMISTRY LETTERS, pp. 783-786, 1985.
The aforementioned process uses a mixed catalyst that is a mixture of the FT synthesis catalyst and the solid acid catalyst such as zeolite as described above, so as to produce lower isoparaffins from syngas in one pass through the mixed catalyst. The resultant lower isoparaffins have a high octane number and are suitable for use as high-performance transportation fuel.
In the process for synthesis of lower isoparaffins using the known FT synthesis reaction as described above, the optimal temperature for the synthesis reaction on a cobalt catalyst as one type of the FT synthesis catalyst is in the range of 240 to 260xc2x0 C., whereas the optimal temperature for the hydrocracking reaction on zeolite as one type of the solid acid catalyst is in the range of 280 to 320xc2x0 C. Thus, there is a great difference in the optimal temperature range between the above two reactions. In other words, the one-pass reaction for synthesis of lower isoparaffins suffers from a mismatch in the optimal temperature range between the FT synthesis catalyst and the solid acid catalyst.
If the synthesis of lower isoparaffins is carried out at a temperature in the range of 280 to 320xc2x0 C., which is the optimal temperature range for the hydrocracking reaction, the selectivity for methane in the FT synthesis reaction may undesirably increase.
If the synthesis of lower isoparaffins is carried out at a temperature in the range of 240 to 260xc2x0 C., which is the optimal temperature range for the FT synthesis reaction, on the other hand, the selectivity for methane may be reduced, but there may arise other problems as follows: the selectivity factor for isoparaffins is reduced due to an insufficient ability of the solid acid catalyst to achieve hydrocracking, and the carbon numbers of hydrocarbons produced in this manner are distributed over an extended or larger range.
It is an object of the invention to provide a process for synthesis of lower isoparaffins from synthesis gas wherein synthesis reactions are conducted at temperatures most suitable for respective types of catalysts so that the selectivity for lower isoparaffins as a target product can be increased.
To accomplish the above and other objects, the invention provides a process for synthesis of lower isoparaffins from synthesis gas that is a mixture of hydrogen and carbon monoxide, comprising the steps of: (1) synthesizing straight chain hydrocarbons in a first stage by contacting the synthesis gas with a Fischer-Tropsch synthesis catalyst that is mixed with a solid acid catalyst for mainly hydrocracking long chain hydrocarbons, and (2) synthesizing isoparaffins in a second stage by contacting the straight chain hydrocarbons synthesized in the first stage, with a mixture of a hydrogenation catalyst for hydrogenating olefins and a solid acid catalyst for hydrocracking and isomerizing the straight chain hydrocarbons.
In the process as described above, the Fischer-Tropsch synthesis catalyst may be cobalt (Co) supported by silica or CoMnO2 prepared by a coprecipitation method.
In the process as described above, the hydrogenation catalyst may be paradium (Pd) or platinum (Pt) supported by silica or active carbon, for example.
In the process as described above, the hydrogenation catalyst may be paradium (Pd) or platinum (Pt) directly supported by, for example, zeolite serving as the solid acid catalyst.
In the process as described above, hydrogen may be added to the second stage in which the isoparaffins are synthesized.
In the process as described above, synthesis of the straight chain hydrocarbons in the first stage may be carried out at a temperature in a range of 240 to 260xc2x0 C., and synthesis of the isoparaffins in the second stage may be carried out at a temperature in a range of 280 to 320xc2x0 C.